This paper evaluates the performance of TiN and Al2O3 single layer coated c
emented carbide tools when threading inclusion-modified 708M40T (En 19T) an
d 817M40T (En 24T) steels at high cutting conditions. Tool wear, failure mo
des, post-threading workpiece properties, micro- and macro-surface alterati
ons and sub-surface microhardness variation after threading were monitored
and used to assess the performance of the threading tools. Results of the t
hreading show that flank wear was the dominant failure mode and tended to i
ncrease rapidly when machining at the top speed of 225 m min(-1) due to the
high temperature generated, which accelerates thermally-related wear mecha
nisms. The Al2O3 coated carbide inserts with K05-K20 substrate generally pe
rformed better than TiN coated grade with P20-P30 substrates, under the cut
ting conditions investigated, because of their superior hardness, density,
transverse rupture strength as well as unalloyed WC fine grained substrate
(1 mu m) in addition to the high hot hardness, excellent chemical stability
and low thermal conductivity of the Al2O3 coating at elevated temperatures
. An equation for tool rejection was also developed during this study based
on the average flank wear (VBb) and growth in thread root (GTR), to establ
ish a scientific basis for assessing the wear of threading tools. (C) 1999
Elsevier Science S.A. All rights reserved.